Diagnostic ultrasound imaging systems provide a comprehensive evaluation of person's health condition. The efficacy of ultrasound techniques have resulted in the widespread acceptance of ultrasound imaging by both patients and physicians. In general, diagnostic ultrasound imaging systems generate images of anatomical structures within the patient by transmitting ultrahigh frequency soundwaves (typically in the order of 3.0-10.0 MHz) and then analyzing the waves reflected from the body structure. The most widely used ultrasonic diagnostic systems display structural information of organs in the form of two-dimensional images of selected cross sections of the organ. Typically, the ultrasound is swept across the organ in the form of a sector scan. The sector scan is ordinarily performed in real time so that images are available to the physician during an examination.
In presently available ultrasound systems, in addition to anatomical information, blood flow information is often provided by utilizing the Doppler principle. A beam of ultrasonic energy is directed toward a blood vessel or other organ in which blood flow information is desired. For example, the placenta wherein the blood vessels between the mother and the embryo are interfaced, but not joined. To use the Doppler principle, the beam of ultrasonic energy is directed toward a blood vessel. Moving blood cells reflect the ultrasound energy and either increase or decrease the frequency of the reflected energy depending on the direction of the blood flow in accordance with the well known Doppler principle.
The magnitude of the frequency shift and the direction of the shift are detected so that the direction of the blood flow may be ascertained. Such Doppler ultrasound apparatus also typically provides the usual anatomical information using conventional diagnostic ultrasound techniques.
The Doppler ultrasound equipment now in use, however, fail to provide for mapping of flow parameters other than average velocity of the flow. Thus, while the analysis of blood flow using ultrasound has found a variety of applications in recent years, it has not been used to perform effective mapping of flow parameters other than velocity.
The use of Doppler to study fetal growth is based on the fact that normal fetal growth depends on an adequate supply of oxygen and nutrients which are generally carried to the fetus by the fetal blood through the umbilical placental circulation throughout pregnancy. The studies of the umbilical placental circulation of the human fetus have been greatly facilitated by the use of the aforementioned Doppler ultrasound and analysis of the flow. Recently, there have been some studies wherein the waveforms of the flow velocity have been studied. However, most of these studies have focused on the umbilical artery. The characteristics of the fetal circulation further downstream to the umbilical artery have seldom been studied and certainly, no effective method of using clinical ultrasound has been developed to augment such studies.
Studies of flow parameter characteristics would also be beneficial to determine whether or not a suspicious mass is a malignant tumor. It is known that tumorous masses are generally accompanied by angiogenesis which results in increased diastolic blood flow. This blood flow can be measured and characterized with spectral Doppler. In the past, attempts using spectral Doppler for characterizing tumors have not proved clinically very successful because the neovascularization involves very small blood vessels that are difficult to detect using two-dimensional real time ultrasound imaging, or even using real time color flow imaging, because of its limited sensitivity to slow weak flows.